1. Field of the Invention
The present invention relates to a zero-cross detection circuit for detecting a point at which an input alternating-current (AC) voltage crosses a predetermined voltage (0 V). More particularly, the present invention relates to a zero-cross detection circuit which is connected to a full-wave rectifying and smoothing circuit powered from a commercial AC power supply for full-wave rectification and smoothing, and which is also connected to a switching regulator for separating and stepping down the output from the full-wave rectifying and smoothing circuit to output a desired DC voltage.
2. Description of the Related Art
FIG. 10 is a circuit diagram of a power supply circuit using a commercial AC power input, in particular, showing a zero-cross detection circuit in the related art for detecting a point at which an input AC voltage crosses zero volts, and a rectifying and smoothing circuit and a switching regulator which are connected to the zero-cross detection circuit.
In FIG. 10, lines Line1 and Line2 are connected to a commercial AC power supply through a filter circuit (not shown). The full-wave rectifying and smoothing circuit is formed of diodes D11, D12, D13, and D14, and a smoothing capacitor C11.
In FIG. 10, the switching regulator which is the self-excitation type is formed of components indicated by Q21, Q22, C21, C22, C31, C32, D31, IC31, R21 to R27, R31 to R35, and PC21. The switching regulator is insulated by a transformer T21, and generates a constant voltage of +24 V.
The zero-cross detection circuit is formed of components indicated by Q41, C41, C42, D41, R41, R43, R44, R45, and PC41. In the zero-cross detection circuit, a low-voltage output terminal of the full-wave rectifying and smoothing circuit is connected to the emitter of the n-p-n transistor Q41, and the resistor R43 is connected between the base and emitter of the transistor Q41. The resistor R43 and the capacitor C41 are connected in parallel with each other, and the resistor R41 is connected between the capacitor C41 and the line Line1.
A half-wave rectifying circuit is formed of the resistors R41 and R43 in the zero-cross detection circuit, and the diode D13, and the output of the half-wave rectifying circuit is applied between the base and emitter of the transistor Q41. If the potential of the line Line1 is higher than the potential of the line Line2, a current flows in the resistor R41; otherwise, no current flows in the resistor R41. The resistances of the resistors R41 and R43 are set to suitable values so that the collector potential in the transistor Q41 can substantially change according to the potential magnitude of the lines Line1 and Line2. The high/low edges of the collector potential in the transistor Q41 correspond to zero crossings, and a zero-cross signal ZEROX is transmitted to the secondary of the transformer T21 via the photocoupler PC41. The capacitor C41 is a capacitor for removing noise, and is not essential to the zero-cross detection circuit.
FIGS. 11A to 11C and 12A to 12C are signal waveforms of the components in the zero-cross detection circuit.
In FIGS. 11A to 11C and 12A to 12C, the x-axis represents time. FIG. 11A shows the potential of the line Line1 with respect to a ground GND, FIG. 11B shows the potential of the line Line2 with respect to the ground GND, and FIG. 11C shows the difference in potential between the lines Line1 and Line2. FIG. 12A shows a current flowing in the resistor R41, FIG. 12B shows an enlarged version of the y-axis in FIG. 12A, and FIG. 12C shows the phototransistor collector potential in the secondary of the photocoupler PC41, that is, the zero-cross signal ZEROX. In the secondary of the transformer T21, the voltage is stepped down from the output (+24 V) of the switching regulator to +3.3 V.
FIG. 13 shows another zero-cross detection circuit in the related art. The zero-cross detection circuits shown in FIGS. 10 and 13 are different from each other in that the zero-cross detection circuit shown in FIG. 13 further includes capacitors C12 and C13. Specifically, in FIG. 13, the capacitors C12 and C13 are connected to the high-voltage output terminal and the low-voltage output terminal of the full-wave rectifying and smoothing circuit, respectively, and the node between the capacitors C12 and C13 is grounded.
In general, for the terminal noise suppression purpose, a capacitor (a so-called Y-capacitor) of approximately several thousand picofarads is connected between a commercial AC power supply line and a ground GND. The capacitors C12 and C13 are Y-capacitors. Although a Y-capacitor may be connected to an input terminal of a full-wave rectifying circuit, it is more effective for the terminal noise suppression purpose to connect a Y-capacitor to an output terminal of a full-wave rectifying circuit. The configuration shown in FIG. 13 is often used.
In the circuit configuration shown in FIG. 13, if Y-capacitors (the capacitors C12 and C13) have a small capacitance or if the commercial AC power supply exhibits a normal waveform, no problem occurs. However, if the Y-capacitors have a large capacitance or if the commercial AC power supply exhibits an undesirable waveform which is not normal, a problem occurs.
FIGS. 14A to 14C are signal waveforms of the components in the zero-cross detection circuit when the Y-capacitors in the circuit shown in FIG. 13 have a relatively small capacitance. Since the waveforms indicating the potential of the line Line1 with respect to the ground GND, the potential of the line Line2 with respect to the ground GND, and the difference in potential between the lines Line1 and Line2 are the same as those shown in FIGS. 11A to 11C, a description thereof is omitted.
FIG. 14A shows a current flowing in the resistor R41, FIG. 14B shows an enlarged version of the y-axis in FIG. 14A, and FIG. 14C shows the phototransistor collector potential in the secondary of the photocoupler PC41, that is, the zero-cross signal ZEROX.
As is apparent from FIGS. 14A to 14C, if the Y-capacitors have a relatively small capacitance, a zero-cross signal ZEROX can be successfully generated.
FIGS. 15A to 15C are signal waveforms of the components in the zero-cross detection circuit when the Y-capacitors in the circuit shown in FIG. 13 have a relatively large capacitance. Since the waveforms indicating the potential of the line Line1 with respect to the ground GND, the potential of the line Line2 with respect to the ground GND, and the difference in potential between the lines Line1 and Line2 are the same as those shown in FIGS. 11A to 11C, a description thereof is omitted.
FIG. 15A shows a current flowing in the resistor R41, FIG. 15B shows an enlarged version of the y-axis in FIG. 15A, and FIG. 15C shows the phototransistor collector potential in the secondary of the photocoupler PC41, that is, the zero-cross signal ZEROX.
As is apparent from FIGS. 15A to 15C, if the Y-capacitors have a large capacitance, the zero-cross signal ZEROX shown in FIG. 15C does not indicate a correct zero-cross point.
In FIG. 15B, the current flowing in the resistor R41 rises in a sine-wave fashion at about 15 msec, thus causing a zero-cross point to be unsuccessfully detected.
This current flows in the Y-capacitors C12 and C13 towards the ground GND.
The potential of either the line Line1 or Line2, whichever is lower, is used as the low-voltage output potential of the full-wave rectifying and smoothing circuit with respect to the ground GND. This exhibits a half-wave rectified waveform. The high-voltage output potential of the full-wave rectifying and smoothing circuit with respect to the ground GND is produced by adding the capacitance potential stored in the capacitor C11 to the low-voltage output potential, and, if a DC component is removed, the output would be equal to the half-wave rectified waveform. A current flowing in the Y-capacitors C12 and C13 is therefore produced by differentiating the voltage of the half-wave rectified waveform.
This current flows in the rectifier diodes D11 to D14 when those diodes are conducting, and otherwise flows in the resistor R41. Since the full-wave rectifying and smoothing circuit is capacitor-input, the rectifier diodes D11 to D14 have a small conduction angle and do not conduct most of the time.
Therefore, a current shown as the waveform between about 15 and 20 msec in FIG. 15B flows in the resistor R41.
FIGS. 16A to 16C and 17A to 17C are signal waveforms of the components in the zero-cross detection circuit when the commercial AC power supply on which the third harmonic is superposed exhibits an undesirable waveform in the circuit shown in FIG. 13.
FIG. 16A shows the potential of the line Line1 with respect to the ground GND, FIG. 16B shows the potential of the line Line2 with respect to the ground GND, and FIG. 16C shows the difference in potential between the lines Line1 and Line2. FIG. 17A shows a current flowing in the resistor R41, FIG. 17B shows an enlarged version of the y-axis in FIG. 17A, and FIG. 17C shows the phototransistor collector potential in the secondary of the photocoupler PC41, that is, the zero-cross signal ZEROX.
FIGS. 18A to 18C and 19A to 19C are signal waveforms of the components in the zero-cross detection circuit when the commercial AC power supply which has a 60xc2x0 phase difference rather than a 180xc2x0 phase difference exhibits an undesirable waveform in the circuit shown in FIG. 13.
FIG. 18A shows the potential of the line Line1 with respect to the ground GND, FIG. 18B shows the potential of the line Line2 with respect to the ground GND, and FIG. 18C shows the difference in potential between the lines Line1 and Line2. FIG. 19A shows a current flowing in the resistor R41, FIG. 19B shows an enlarged version of the y-axis in FIG. 19A, and FIG. 19C shows the phototransistor collector potential in the secondary of the photocoupler PC41, that is, the zero-cross signal ZEROX.
As is apparent from FIGS. 16A to 19C, if the commercial AC power supply exhibits an undesirable waveform, a correct zero-cross signal ZEROX is not generated.
These phenomena are also caused by a current flowing in the Y-capacitors C12 and C13 towards the ground GND.
If the harmonic is superposed on the commercial AC power supply, the harmonic component is also superposed on the output potential of the full-wave rectifying and smoothing circuit with respect to the ground GND. The harmonic distortion causes a current shown as the waveform between 15 and 20 msec in FIG. 17B to flow in the resistor R41.
If the line Line1 is 180xc2x0 out of phase with respect to the line Line2 of the commercial AC power supply, the difference in potential between the lines Line1 and Line2 is phase-shifted with respect to the output potential of the full-wave rectifying and smoothing circuit with respect to the ground GND, thus causing a current shown as the waveform between about 15 and 20 msec in FIG. 19B to flow in the resistor R41. In FIG. 19B, the current flowing in the resistor R41 is zero at about 18 msec because the rectifier diodes D11 and D14 are conducting, and the current at this time is supplied to the Y-capacitors C11 and C12 via the diode D14.
Accordingly, it is an object of the present invention to provide a zero-cross detection circuit of the half-wave rectifier type, which is correctly operated when a Y-capacitor is provided after a full-wave rectifying and smoothing circuit for the terminal noise suppression purpose and which reliably generates a zero-cross signal when a commercial AC power supply exhibits an undesirable waveform.
A zero-cross detection circuit according to the present invention is connected to a power supply device which includes a full-wave rectifying and smoothing circuit, two capacitors, and a switching regulator. The full-wave rectifying and smoothing circuit is powered via first and second lines from a commercial AC power supply for full-wave rectification and smoothing. The two capacitors have first ends connected to high-voltage and low-voltage output terminals of the full-wave rectifying and smoothing circuit, respectively, and second ends connected to a ground. The switching regulator separates and steps down the output from the full-wave rectifying and smoothing circuit to output a desired DC voltage. The zero-cross detection circuit according to the present invention includes a transistor of which the emitter is connected to the low-voltage output terminal of the full-wave rectifying and smoothing circuit for outputting a zero-cross detection signal from the collector; a first resistor connected between the base and emitter of the transistor; a second resistor connected between the first line and the base of the transistor; and a third resistor connected between the second line and the emitter of the transistor.
Preferably, the third resistor has substantially the same resistance as that of the second resistor.
Therefore, the zero-cross detection circuit according to the present invention can be correctly operated when the above-noted Y-capacitors are provided after a full-wave rectifying and smoothing circuit for the terminal noise suppression purpose. Furthermore, if the commercial AC power supply exhibits an undesirable waveform, the zero-cross detection circuit can reliably generate a zero-cross signal.
Preferably, the zero-cross detection circuit includes a diode connected between the base and emitter of the transistor in such a manner that the anode of the diode is connected to the emitter of the transistor.
Also, a diode may be connected between the first line and the second resistor in such a manner that the anode of the diode is connected to the first line. The second resistor and the diode may be replaced.
This arrangement prevents a reverse bias from being applied between the base and emitter of the transistor if noise is present in the commercial AC power supply. Additionally, another diode may be connected between the second line and the third resistor in such a manner that the anode of the second diode is connected to the second line.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.